Add dormant user period setting to application settings

What does this MR do and why?

This is the first in a series of Merge requests which will allow an administrator to change the value of the number of days that need to elapse before a user is considered "dormant".

This adds the column to the application settings table with a default of 90 which is the current hardcoded value.
The follow-on merge requests will add the UI elements and model updates needed to utilize it.

Relevant GitLab issue: #336747 (closed)

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